Establishing a stratigraphic model of the subsurface at a given location serves to discover its structure, i.e. the nature, the position, and the shape of the various geological strata from which it is built up.
When seismic measurements are taken, the seismic signal picked up by a detector is the response of the variations in acoustic impedance of the geological formations to an emitted sound wave. The acoustic impedance of a formation is defined as being the product of the density of the medium multiplied by the propagation velocity of the seismic wave through the medium.
The acoustic impedance profile at a given location corresponds mathematically to a sequence of interface surfaces and the problem is to define the number N of such interfaces, their geometrical positions, and their values of acoustic impedance A={a.sub.i (n),n=1, N}. In equivalent manner, the impedance profile is defined by a sequence of reflection coefficients R.sub.i ={r.sub.i (n),n=1,N} at the interfaces, with values being given by the equation: EQU r.sub.i (n)=[(a.sub.i (n)-a.sub.i (n-1)]/[(a.sub.i (n)+a.sub.i (n-1)]
In order to obtain the acoustic impedance profile at a given location, borehole logging measurements are taken or else downhole seismic measurements are taken.
Borehole logging and downhole seismic measurements can only be performed when a drilled well is available.
Downhole seismic measurements consist in placing receivers at spaced-apart intervals along the borehole A source for emitting sound waves is placed on the surface in the same zone as the borehole.
Whenever the source emits sound, the emitted energy propagates through the ground and encounters successive geological strata (or formations) These strata act as semitransparent reflectors which reflect a portion of the energy. When the reflectors are located beneath the receivers, the reflected energy is picked up and recorded by each of the receivers, with the energy reaching each receiver after a time delay related to the distance of the receiver from the source and to the propagation velocity through the strata. Each receiver also receives a direct wave coming from the source by transmission through the various strata.
A multiplicity of measurements are taken by moving the source over the surface or by moving the receivers along the borehole, or vice versa.
Logging measurements consist in measuring physical parameters along the borehole These measurements make it possible to obtain a high resolution curve representing the nature of the rock along the borehole. By studying such logs, it is possible to define the probable positions of the interfaces, i.e. of the locations where there are changes in the nature of the strata, and these positions can be determined with very good precision along the borehole since, firstly the measurements are performed at a sampling interval which is smaller than the sampling interval used for seismic measurements, and secondly because the frequency band used is narrower than that used for seismic measurements with the signal emitted for seismic measurements being incapable of obtaining measurements with a high degree of precision.
When an acoustic impedance profile is searched at a given location from a seismic section, a conventional technique is to invert the seismic signals, and in particular the vertical seismic profile (VSP) signals.
Before performing any downhole seismic measurements or any borehole logging, seismic measurements are performed on the surface in order to establish a seismic section. A seismic section is a picture of the subsurface made from all the seismic traces recorded by a set of receivers when a seismic wave is emitted by a source, with the source and the receivers being spaced apart on the surface of the ground.
The underground geological strata act as semitransparent reflectors with a portion of the incident waves being reflected at each interface between two strata while the remainder of the energy is transmitted towards lower strata.
A given geological stratum reflects a portion of the received wave to each of the receivers on the surface Each receiver picks up the energy reflected from each stratum and each receiver records the energy reflected by a given stratum after a time delay related to its distance from the source.
The purpose of all these measurements, i.e. downhole seismic measurements, surface seismic measurements, and borehole logging, is to establish the values of the reflection coefficients at the interfaces between the various strata together with the positions of said interfaces in order to deduce the corresponding acoustic impedances, with the impedance of stratum being a function of its nature. It is thus possible to obtain a stratigraphic image of the subsurface.
As already mentioned, borehole logging serves to obtain the acoustic impedance profile at a borehole with very high precision.
Downhole seismic measurements give acoustic impedance profiles with fairly good precision over a small lateral extent.
Surface seismic measurements give acoustic impedance profiles over a wide lateral extent but with fairly low precision.
The object of the present invention is to provide a method of obtaining successive acoustic impedance profiles along a seismic section with high precision close to that obtained with borehole logging, and with a lateral extent similar to that obtained with surface seismic measurements, in order to establish a stratigraphic model of the subsurface under consideration.